Transparent conductive film and coating composition therefor

ABSTRACT

The disclosed is a transparent conductive film that includes a matrix and carbon fibrous structures added to the matrix, wherein the carbon fibrous structures comprise carbon fibers, each having an outside diameter of 15-100 nm, and wherein the carbon fibrous structures each comprise a granular part at which two or more carbon fibers are bound to each other, and wherein the granular part is concurrently produced in a growth process for the carbon fibers. When the transparent conductive film is formed at a thickness of 0.1-5 μm on a glass substrate, it shows a surface resistivity of not more than 1.0×10 12 Ω/□, and a total light transmittance of not less than 30%. A coating composition for the conductive transparent film is prepared by using a media mill equipped with beads having an average diameter of 0.05-1.5 mm to disperse the carbon fibrous structures into the liquid resinous composition.

CROSS REFERNCE TO RELATED APPLICATIONS

This claims priority of Japanese Patent Application No. 2005-132691, filed on Apr. 28, 2005, the disclosure of which, including the specification, claims, drawings and summary, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a transparent conductive film and a coating composition for the transparent conductive film. More particularly, this invention relates to a transparent conductive film that has a good transparency and a high conductivity, as well as to a coating composition for forming the transparent conductive film.

Background Art

Transparent conductive films have been utilized as electrode materials for liquid crystal display devices, organic electroluminescent (EL) devices, and various other electronic devices. They have also been used for the purpose of removing static electricity from transparent members in order to prevent sticking of dusts to such members, through which viewing of the other sides is desired, such as partitions of clean rooms and inspection windows of various test equipment.

Inorganic acid type materials such as ITO, IZO, etc., and metal evaporated films have been developed as transparent conductive films in wide use. However, there are several restrictions on these materials, such as difficulty in controlling their electrical characteristics, and limited range of applicable substrate materials.

Another type of conductive film whose conductivity is imparted by particles of a certain material, such as metal, metal oxide, carbon, etc., added to the matrix of the film is also known. Furthermore, a transparent conductive resin board comprising long carbon fibers blended with a thermoplastic resin that acts as a matrix has been proposed in JP-2001-62952A and JP-2004-195993A. Other transparent conductive resin boards have also been proposed in JP-2004-230690A, wherein carbon nanotubes are added to a thermoplastic resin under the condition that the carbon nanotubes are dispersed in a mutually independent manner, or as bundles wherein each bundle is composed of some carbon nanotubes and the bundles are dispersed mutually independent of each other. The related parts of JP-2001-62952A, JP-2004-195993A and JP-2004-230690A are incorporated herein by reference.

In the transparent conductive resin boards shown in JP-2001-62952A, JP-2004-195993A and JP-2004-230690A, however, it is difficult to disperse the fibers uniformly throughout the thermoplastic resin matrix since the long carbon fibers or carbon nanotubes must be mixed in single-fiber form with the thermoplastic resin matrix. Therefore, the resulting conductivity of the boards can hardly be expected to be uniformly inplane. When kneading force is increased in order to enhance the dispersibility of the carbon fibers into the matrix, it occurs the problem that the fibers would be cut into shreds. Consequently it is necessary to add more fibers in order to achieve a predetermined conductivity. The increased amount of fibers will result in a reduction of transparency of the board.

BRIEF SUMMARY OF THE INVENTION

Therefore, this invention aims to provide a new transparent conductive film and a coating composition therefor capable of solving above mentioned problems in the arts. This invention also aims to provide a transparent conductive film and a coating composition therefor, which possesses improved and well controllable electrical properties by adding a small amount of additive or filler without damaging the characteristic of the matrix, while maintaining a good transparency of the film.

As a result of diligent study for solving the above problems, the inventors have found that the followings are effective at improving the electrical properties of a matrix even at a limited additive amount, and finally accomplished the present invention:

To adapt carbon fibers having a diameter as small as possible;

To make a sparse structure of the carbon fibers, in which the fibers are bound to each other tightly so that the fibers do not behave individually and their sparse state are sustained in the resin matrix;

To adapt as carbon fibers per se ones which are designed to have a minimum amount of defects; and

To use a particular dispersion treatment capable of dispersing carbon fibrous structures throughout a matrix uniformly without destroying the fibrous structures.

The present invention to solve the above mentioned problems is, therefore, a transparent conductive film, which comprises a matrix resin and carbon fibrous structures added to the matrix, wherein the carbon fibrous structures are comprised of carbon fibers each having an outside diameter of 15-100 nm, and wherein each carbon fibrous structure further comprises a granular part at which two or more carbon fibers are bound to each other in a state that the concerned carbon fibers are outwardly elongated therefrom, and wherein the granular part is produced in a growth process for the carbon fibers.

In an embodiment of a transparent conductive film according to the present invention, it is disclosed that carbon fibrous structures are added to a matrix resin at an amount in the range of 1 to 25 parts by weight based on 100 parts by weight of the matrix.

In another embodiment of a transparent conductive film according to the present invention, it is disclosed that the transparent conductive film has a surface resistivity of not more than 1.0×10¹²Ω/□, and a total light transmittance of not less than 30%, when the transparent conductive film is formed at a thickness of 0.1-5 μm on a glass substrate.

In another aspect, the present invention to solve the above mentioned problems is a coating composition for a transparent conductive film. The coating composition comprises a liquid resinous composition, including a resin as a non-volatile vehicle, and carbon fibrous structures dispersed into the resinous composition, wherein the carbon fibrous structures are comprised of carbon fibers each having an outside diameter of 15-100 nm, and wherein each carbon fibrous structure further comprises a granular part at which two or more carbon fibers are bound to each other in a state that the concerned carbon fibers are outwardly elongated therefrom, and wherein the granular part is produced in a growth process for the carbon fibers.

In an embodiment of a coating composition for a transparent conductive film according to the present invention, it is disclosed that the carbon fibrous structures are added at an amount in the range of 1 to 25 parts by weight based on 100 parts by weight of the liquid resinous composition.

In another embodiment of a coating composition for a transparent conductive film according to the present invention, it is disclosed that the coating composition is prepared by using a media mill equipped with beads having an average diameter of 0.05-1.5 mm to disperse carbon fibrous structures into a liquid resinous composition.

In a further embodiment of a coating composition for a transparent conductive film according to the present invention, it is disclosed that the coating composition is prepared by using a high-shear type distributor prior to the dispersion treatment using the media mill.

According to embodiments of the present invention, each of the carbon fibrous structures to be added to a matrix resin as an electrical conductivity imparting agent has a specific configuration in which some carbon fibers are bound to each other tightly by a granular part produced in a growth process for the carbon fibers wherein the concerned carbon fibers are outwardly elongated from the granular part. Such carbon fibrous structures can disperse easily into a matrix resin upon adding, while maintaining their sparse structure. Even when they are added at a small amount to the matrix, they can be distributed uniformly throughout the matrix. Therefore, with respect to electrical conductivity, it is possible to obtain good electrical conductive paths throughout the matrix even with a small dosage of added fibrous structures, thereby improving the electrical conductivity adequately and controllably of the conductive film. With respect to transparency of the film, a high degree of transparency can be maintained since the carbon fibrous structures can be uniformly distributed throughout the matrix.

Further, on preparing a coating composition for a transparent conductive film according to the present invention, using a media mill equipped with beads of a prescribed mean diameter, it is possible to achieve a good and uniform dispersion without adding any dispersion stabilizer such as a surfactant. In this way, breakdown of the carbon fibrous structures is avoided, and a transparent conductive film having good properties can be prepared with ease.

In addition, by subjecting the mixture of resinous composition and carbon fibrous structures to a dispersion treatment using a high-shear type distributor before dispersion treatment using a media mill, a more homogeneous distribution can be attained, which, in turn, leads to improve film properties on making the film film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron micrograph (TEM photo) of an intermediate for the carbon fibrous structure, which is used for a transparent conductive film according to embodiments of the present invention;

FIGS. 2A and 2B are transmission electron micrographs (TEM) of a carbon fibrous structure, which is used for a transparent conductive film according to embodiments of the present invention;

FIG. 3 is an X-ray diffraction chart of a carbon fibrous structure and an intermediate of the carbon fibrous structure, which are used for a transparent conductive film according to embodiments of the present invention and an intermediate thereof;

FIG. 4 is Raman spectra of a carbon fibrous structure and an intermediate of the carbon fibrous structure, which is used for a transparent conductive film according to embodiments of the present invention and an intermediate thereof;

FIG. 5 is an optical microphotograph illustrating dispersion condition of the carbon fibrous structures in an embodiment of a transparent conductive film according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference to some embodiments, which are disclosed only for the purpose of facilitating the illustration and understanding of the present invention, and are not to be construed as limiting.

A transparent conductive film according to embodiments of the present invention is characterized by the inclusion and dispersion of carbon fibrous structures in a thermosetting matrix resin, wherein each of the carbon fibrous structures has a specific configuration to be described later.

A carbon fibrous structure to be combined with a transparent conductive film according to embodiments of the present invention is, as shown in TEM photos of FIG. 2A and 2B, comprised of carbon fibers each having an outside diameter of 15-100 nm, and wherein a granular part at which several carbon fibers are bound to each other, wherein the concerned carbon fibers are outwardly elongated from the granular part.

The reason for ranging the outside diameter of a carbon fiber between 15 nm and 100 nm is because when the outside diameter of the carbon fiber is less than 15 nm, the cross-section of the carbon fiber does not have a polygonal figure, as will be discussed later. For a physical property of a carbon fiber, the more the number of the carbon fibers increase per unit quantity and the smaller the diameter it has, the longer its length in the axial direction will be. Longer length and smaller diameter is generally associated with an enhancement in electrical conductivity. Thus, carbon fibrous structures having outside diameters exceeding 100 nm is not preferred to use as conductivity imparting agent in a matrix such as a resinous material, etc. Particularly, it is more desirable for a carbon fiber to have a diameter in the range of 20-70 nm. Carbon fibers that have diameters within the preferred range and tubular graphene sheets layered orthogonal to the axis, i.e., being of the multilayer type, can have high bending stiffness and ample elasticity. In other words, such carbon fibers would have the property of being easy to restore to their original shapes after undergoing any deformation. Therefore, they tend to take a sparse structure in a matrix such as a resin after mixing with the matrix material, even if they have been compressed.

Incidentally, annealing at a temperature of not less than 2400° C. would cause the carbon fibers to form polygonal cross-sections. Additionally, annealing also narrows the spacing between the layered graphene sheets, which can increase the true density of the carbon fibers from 1.89 g/cm³ to 2.1 g/cm³. Therefore, annealed carbon fibers can become denser and have fewer defects in both the stacking direction and the surface direction of the tubular graphene sheets which comprise a carbon fiber, with the result that flexural rigidity (EI) is enhanced.

Additionally, it is preferable that the outside diameter of a carbon fiber varies along the axial direction of the fiber. Fibers not having certain diameters but having variable diameters along their axial direction are expected to have a kind of anchoring effect at the interface between the fiber and the matrix material such as a resin, thus restraining migration of the carbon fibrous structure in the matrix and improving the dispersion stability.

Then, in a carbon fibrous structure according to embodiments of the present invention, the carbon fibers would have predetermined outside diameters, be configured three dimensionally, and be bound to each other by a granular part produced in a growth process for the carbon fibers, wherein the carbon fibers are outwardly elongated from the granular part. Since the multitude of carbon fibers are not only entangled with each other, but also bound to each other tightly at the granular part, they do not disperse as single fibers, but as bulky carbon fibrous structures when added to the matrix such as resin. Since the carbon fibers are bound together by a granular part, produced in the growth process for the carbon fibers, in a carbon fibrous structure according to embodiments of the present invention, the carbon fibrous structure itself can have superior properties such as electrical property. For instance, a carbon fibrous structure according to the present invention exhibits an extremely small electrical resistance under a certain pressed density, as compared with that of a simple aggregate of carbon fibers and that of a carbon fibrous structure in which the carbon fibers are fixed at the contacting points with a carbonaceous material or carbonized the resultant carbon fibers after their synthesis. Thus, when a carbon fibrous structure in accordance with embodiments of the invention is added and distributed into a matrix, it can form a good conductive path in the matrix.

Further, although not specifically limited, it is preferred that the diameter of the granular part is larger than the outside diameter of a carbon fiber as shown in FIGS. 2A and 2B. When the granular part (the binding site of the carbon fibers) has an amply large particle diameter, the binding force between the outwardly elongated carbon fibers and the granule is enhanced, and thus, even when the carbon fibrous structures are exposed to a relatively high shear stress upon blending into a matrix such as a resin, they can be dispersed as three dimensionally structures into the matrix. The “particle diameter of the granular part” used herein is the value measured by assuming that the granular part is biding site of each carbon fibers.

Further, a carbon fibrous structure to be used in the present invention can have a bulky form, in which the carbon fibers are sparsely configured since the carbon fibrous structure is comprised of carbon fibers that are bound to each other by a granular part, wherein the carbon fibers are outwardly elongated from the granular part as mentioned above. Specifically, it is desirable that the bulk density thereof is in the range of 0.0001-0.05 g/cm³, more preferably, 0.001-0.02 g/cm³. When the bulk density exceeds 0.05 g/cm³, the improvement in the physical properties of a matrix resin would be more difficult to attain with a small dosage.

Further, a carbon fibrous structure to be used in the present invention can itself have good electrical properties, since the carbon fibers comprising the three dimensionally structure are bound to each other by a granular part produced in a growth process for the carbon fibers as mentioned above. For instance, it is desirable that a carbon fibrous structure according to embodiments of the present invention shows a particle's resistance of not more than 0.02 Ω·cm, as determined under a certain pressed density (0.8 g/cm³), and more preferably, 0.001 to 0.010 Ω·cm. If the particle's resistance exceeds 0.02 Ω·cm, it may be more difficult to form a good conductive path when the structures are added to a matrix resin.

In order to enhance the strength and electrical conductivity of a carbon fibrous structure according to embodiments of the present invention, it is desirable that the graphene sheets making up the carbon fibers have a minimum number of defects, and more specifically, for example, the I_(D)/I_(G) ratio of the carbon fibers as determined by Raman spectroscopy is not more than 0.2, more preferably, not more than 0.1. Incidentally, in Raman spectroscopic analysis, if the sample is a large single-crystal graphite, only the peak (G band) at 1580 cm⁻¹ is observed. When the crystals are of finite minute sizes or have any lattice defects, a peak (D band) at 1360 cm⁻¹ can appear. Therefore, when the ratio between D band and G band (R═I₁₃₆₀/I₁₅₈₀=I_(D)/I_(G)) is within the certain value as mentioned above, it is possible to infer that there is little defect in the graphene sheets.

Further, it is desirable that a carbon fibrous structure according to embodiments of the present invention has a combustion initiation temperature in air of not less than 750° C., preferably, 800° C.-900° C. Such a high thermal stability can be attributed to the facts that the fibers have very little defect and have predetermined outside diameters.

It is not limited to, but a carbon fibrous structure of the above desirable form may be prepared as follows.

Basically, an organic compound such as a hydrocarbon is chemical thermally decomposed through the CVD process in the presence of ultraminute particles of a transition metal as a catalyst in order to obtain a fibrous structure (hereinafter referred to as “intermediate”). The intermediate thus obtained is then subjected to a high temperature heating treatment.

For raw material organic compounds, hydrocarbons such as benzene, toluene, xylene; carbon monoxide (CO), and alcohols such as ethanol may be used. It is preferable, but not limited, to use as carbon sources at least two carbon compounds having different decomposition temperatures. Incidentally, the phrase “at least two carbon compounds” used herein does not only mean using two or more kinds of raw materials, but also means using one kind of a raw material that can effect a reaction such as the hydrodealkylation of toluene or xylene during the course of synthesis reaction for the fibrous structure to produce an intermediate, which can function as at least two kinds of carbon compounds having different decomposition temperatures in the subsequent thermal decomposition process.

Inert gases such as argon, helium, xenon; and hydrogen may be used as an atmospheric gas.

A mixture of transition metal such as iron, cobalt, molybdenum; or transition metal compounds such as ferrocene, metal acetate; and sulfur or a sulfur compound such as thiophene, ferric sulfide; may be used as a catalyst.

The intermediate may be synthesized using a CVD process of hydrocarbon or other compounds conventionally used in the art. A typical CVD process includes the following steps: gasifying a mixture of a raw material organic compound and a catalyst, supplying the gasified mixture into a reaction furnace along with a carrier gas such as hydrogen gas, etc., and performing thermal decomposition at a temperature in the range of 800° C.-1300° C. By following such a synthesis procedure, one obtains an aggregate of several to several tens of centimeters in size composed of plural carbon fibrous structures (intermediates), each of which shows a sparse three dimensional configuration, wherein fibers having 15-100 nm in outside diameter are bound to each other by a granule that has grown around a catalyst particle as the nucleus.

The thermal decomposition reactions of hydrocarbon raw materials mainly occur on the surface of catalyst particles or on a surface of a granule grown around a catalyst particle nucleus. When recrystallization of carbons created from the decomposition reaction progresses in a constant direction from the catalyst particles or the granule, fibrous growth of carbon may be achieved. To obtain a carbon fibrous structure according to embodiments of the present invention, however, the balance between decomposition rate and growing rate is varied intentionally. For instance, as mentioned above, using as carbon sources at least two kinds of carbon compounds having different decomposition temperatures may allow the carboneous material to grow three dimensionally around a granule as a centre, rather than growing in one dimensional direction. Three dimensional growth of carbon fibers depends not only on the balance between the decomposition rate and the growing rate, but also on the crystal face's selectivity of a catalyst particle, residence time within the reaction furnace, temperature distribution in the furnace, etc., and the balance between the decomposition rate and the growing rate is affected not only by the kind of the carbon source as mentioned above, but also by the reaction temperature, gas temperature, etc. In general, when the growing rate is larger than the decomposition rate, the carbon material tends to grow in fibrous configuration, whereas when the decomposition rate is larger than the growing rate, the carbon material tends to grow into peripheral directions of the catalyst particle. Accordingly, by changing the balance between the decomposition rate and the growing rate intentionally, it is possible to control the growth of a carbon material in multi-directions rather than in a certain single direction, and to form a three dimensional structure according to embodiments of the present invention.

In order to form the above mentioned three dimensional configuration, wherein the fibers are bound to each other by a granule, it is desirable to optimize the reaction compositions (catalyst, etc.), the residence time in the reaction furnace, the reaction temperature, the gas temperature, etc.

The intermediate obtained by heating the gaseous mixture of a catalyst and hydrocarbons at a constant temperature in the range of 800° C.-1300° C. typically takes a structure resembling some patch-like sheets of carbon atoms laminated together (and being still in half-raw or incomplete condition). When analyzed with Raman spectroscopy, a large D band and many defects are observed.

The intermediate thus obtained includes unreacted raw materials, nonfibrous carbide, tar moiety and catalyst metal. In order to remove such residues to produce the intended carbon fibrous structure with few defects, the intermediate is subjected to a high temperature heating treatment at 2400-3000° C. using a proper method.

For instance, the intermediate may be first heated at 800-1200° C. to remove unreacted raw materials and volatile flux such as tar moiety, and thereafter annealed at a high temperature of 2400-3000° C. to produce the intended structure and to vaporize the catalyst metal that has been included in the fiber concurrently. In this process, it may add a reducing gas and carbon monoxide gas of a small amount into the inert gas atmosphere to protect the material structure.

By annealing the intermediate at a temperature of 2400-3000° C., the patch-like sheets of carbon atoms are rearranged so as to associate mutually and form multiple graphene sheet-like layers.

Either before or after such a high temperature heating treatment, the aggregates may be crushed in order to obtain carbon fibrous structures that have an area-based circle-equivalent mean diameter of several centimeters. The obtained carbon fibrous structures may then be pulverized in order to obtain carbon fibrous structures that have a predetermined area-based circle-equivalent mean diameter of 50-100 μm. Pulverizing directly without crushing is also permissible. Aggregates comprising plural carbon fibrous structures according to embodiments of the present invention may also be treated to adjust their shapes, sizes, or bulk densities to ones suitable for using. More preferably, in order to utilize effectively the structures formed from the reaction described above, annealing should be performed on structures that are in a state of low bulk density (i.e., the state in which the fibers are extended as much as they can and the voidage is amply large), which may contribute to improve the electrical conductivity of a matrix resin.

Next, as resins to be used as a matrix of a transparent conductive film according to embodiments of the present invention, any of various thermoplastic resins and thermosetting resins, as well as other natural resins or modified resins therefrom, for example, may be used. Among them, a resin of thermosetting type may be desirable from the stand point of easy film coating.

With respect to a transparent conductive film according to embodiments of the present invention, the amount of carbon fibrous structures to be mixed into a resin is not particularly limited. For satisfactory good transparency and conductivity, however, it is preferable in general that the carbon fibrous structures are added and dispersed into the resin at an amount of 1-25 parts by weight based on 100 parts by weight of the resin. A transparent conductive film having a mixing amount as mentioned above would have a surface resistivity of not more than 1.0×10¹²Ω/□, and a total light transmittance of not less than 30%, when the transparent conductive film is formed at a thickness of 0.1-5 μm on a glass substrate. In addition, the haze of the transparent conductive film would become not more than 30%.

When used in a particular application such as a transparent electrode material, it is more preferable that carbon fibrous structures are added and dispersed into a resin at an amount of 10-25 parts by weight based on 100 parts by weight of the resin. In this case, a surface resistivity of 10¹-10⁴Ω/□, and a total light transmittance of not less than 50% is expected. In another particular use such as in anti-static window parts, it is more preferable that carbon fibrous structures are added and dispersed into a resin at an amount of 1-10 parts by weight based on 100 parts by weight of the resin. In this case, a surface resistivity of 10⁴-10¹²Ω/□, and a total light transmittance of not less than 30% is expected.

A coating composition for preparing an aforementioned transparent conductive film according to embodiments of the present invention will be described below.

A coating composition according to embodiments of the present invention comprises a liquid resinous composition including a resin as a non-volatile vehicle, and carbon fibrous structures, each having above mentioned specific structure dispersed into the liquid resinous composition.

Carbon fibrous structures used in this coating composition are the same as described above in detail.

A liquid resinous composition to be used in embodiments of this invention may involve various types of liquid resinous compositions, such as water- or oil-based coating compositions, ink compositions, as well as other various coating compositions, in which a resin as a non-volatile vehicle is dissolved in a solvent or dispersed into a dispersant. As for the resin ingredient, various organic compounds such as thermoplastic resins, thermosetting resins, as well as natural resins and modified resins thereof are usable. For example, acrylic type resins such as aqueous acrylic, acrylic lacquer; ester type resins such as alkyd resins, various modified alkyd resins, unsaturated polyesters; melamin type resins; urethane type resins; epoxy type resins, and other resins such as polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polystyrene, polyamide, phenol resin, furan resin, xylene formaldehyde resin, urea resin, and etc., are concretely examples, but these are not limited examples. Then, depending on the kind of resin ingredients used, a liquid resinous composition can be into different types such as baking type, cold setting type, etc.

The liquid to be used as solvent or dispersion medium in a liquid resin composition is also not particularly limited, and may be selected properly in accordance with the kind of the resin ingredients to be used. For example, as liquids, water; alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, butyl alcohols, allyl alcohols; glycols or their derivatives such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycols, polypropylene glycols, diethylene glycol monoethyl ether, polypropylene glycol monoethyl ethers, polyethylene glycol monoallyl ethers, polypropylene glycol monoallyl ethers; glycerol or its derivatives such as glycerol, glycerol monoethyl ether, glycerol monoallyl ether; amides such as N-methyl pyrrolidone; ethers such as tetrahydorofuran, dioxane; ketones such as methyl ethyl ketone, methyl isobutyl ketone; hydrocarbons such as liquid paraffins, decane, decenes, methyl naphthalenes, decalin, kerosene, diphenyl methane, toluene, dimethyl benzenes, ethyl benzenes, diethyl benzenes, propyl benzenes, cyclohexane, partially hydrogenated triphenyl; silicone oils such as polydimethyl siloxanes, partially octyl-substituted polydimethyl siloxanes, partially phenyl-substituted polydimethyl siloxanes, fluorosilicone oils; halogenated hydrocarbons such as chlorobenzenes, dichlorobenzenes, bromobenzenes, chlorodiphenyls, chlorodiphenyl methanes; fluorides; and ester compounds such as ethyl benzoate, octyl benzoate, dioctyl phthalate, trioctyl trimellitate, dibutyl sebacate, ethyl(meth)acrylate, butyl(meth)acrylate, dodecyl(meth)acrylate, etc. may be used.

In a coating composition according to embodiments of the present invention, the amount of the above mentioned carbon fibrous structures to be mixed with a resin composition is not particularly limited, and may be determined properly in view of any requirement, for example, a requirement for certain electrical characteristics of a transparent conductive film. For instance, 1-25 parts by weight of the carbon fibrous structures may be added to 100 parts by weight of the resin composition. For any amount within the above additive range, it is possible to prepare a composition in which carbon fibrous structures are uniformly distributed.

Incidentally, the coating composition according to embodiments of the present invention may include various known additives, such as, coloring agents involving pigments or dyes, various kinds of stabilizers, antioxidants, ultraviolet absorbers, flame retardants, and solvents, unless it disturb the purpose of embodiments of the present invention.

A coating composition for the transparent conductive film according to embodiments of the present invention may be prepared as a highly dispersed system since the carbon fibrous structures to be used each have a sparse structure and, therefore, may have good dispersibility. More preferably, it is desirable to prepare the coating composition by using a media mill, especially, a media mill equipped with beads having a mean diameter of 0.05-1.5 mm, in order to disperse carbon fibrous structures throughout the composition. Specifically, before dispersion treatment using such a media mill, it may be advantageous to subject the mixture to a dispersion treatment using a high-shear type distributor as will be explained in detail below.

When the particle diameter of beads to be used for the media mill is too small, there is a concern that carbon fibrous structures may be broken into more minute pieces. Another concern is that dispersion of carbon fibrous structures may not progress sufficiently because kinetic energies of the beads become too small. Further, handling of the beads also becomes difficult. Therefore, it is desirable that the average diameter of the beads is not less than 0.05 mm, preferably not less than 0.5 mm. On the other hand, when the particle diameter of the beads to be used is too large, inadequate milling applied to carbon fibrous structures becomes the major concern because the number of beads per unit volume is decreased with increasing diameter and, therefore, lowering the dispersion efficiency. As a result, there is a possibility that carbon fibrous structures in the composition may have a large aspect ratio, and the liquidity for the paint or coating agent cannot be expected. Therefore, it is desirable that the average diameter of the beads is not more than 1.5 mm, preferably not more than 1.0 mm.

The beads material as a dispersion media to be used for the media mill is not specifically limited. For example, alumina, zirconia, steel, chrome steel, glass, and etc., may be used. Among them, zirconia beads are preferable, considering the possible existence of impurities in the product and the magnitude of kinetic energy, which is dependent on the specific gravity of the beads material.

The shape of the beads is not specifically limited, but, in general, globular or sphere beads are used.

The type of the media mill to be used is not specifically limited, and any known media mill can be used. For example, various known ATWRITERs, sand mills, ball mills, etc. can be used.

Incidentally, the filling rate of the beads in the vessel of a mill may be determined in accordance with the configuration, etc. of the vessel and stirrer, and is not specifically limited. If the rate is too low, however, there is a concern that the mill may not deliver adequate milling or cutting forces to carbon fibrous structures. On the other hand, if it is too high, the concern is that the high driving force for rotating the mill is needed. Furthermore, because of the increase in the beads' abrasion, there is a concern that contamination in the composition becomes worse. Therefore, it is desirable that the filling rate of the beads is set at 70-85% of the effective volume of the vessel.

Operating conditions such as processing time, axis rotation number, internal pressure in the vessel, motor load, etc., may be varied depending on the amount of carbon fibrous structures to be mixed, the characteristics of the resin into which the carbon fibrous structures are dispersed, particularly, viscosity and compatibility of the resin with the carbon fibrous structure. Thus, the specific conditions should be set appropriately according to the purpose.

A preferable example of a high-shear type distributor to be used before the dispersion treatment using a media mill is a mixer that includes a stirring wheel capable of high speed rotation, and a vessel whose inner peripheral surface is set to be closely adjacent to the outer peripheral surface of the stirring wheel. During operation, the wheel is rotated at a tip speed of not less than 30 m/sec in order to force the liquid to be pressed as a thin film against the inner peripheral surface of the vessel by the centrifugal force. The thin film is allowed to contact the tips of the wheel to perform stirring of the liquid. Other in-line rotor and stator type mixers may also be used preferably. One example of a desirable high-shear type distributor is the T.K. FILMICS® manufactured by TOKUSHU KIKA KOGYO CO., LTD.

Alternatively, any of other high-shear type distributors, such as T.K LABO DISPER, T.K. PIPELINE MIXER, T.K. HOMOMIC LINE MILL®, T.K. HOMO JETTOR, T.K. UNI-MIXER, T.K. HOMOMIC LINE FLOW®, T.K. AGI HOMO DISPER (manufactured by TOKUSHI KIKA KOGYO Co., Ltd.), homogenizer POLYTRON® (manufactured by KINEMATICA AG), homogenizer Physcotron (manufactured by Microtec Co., Ltd.), BIOMIXER(manufactured by Nippon Seiki Co., Ltd.), turbo type stirring machine (manufactured by KODAIRA SEISAKUSHO Co., Ltd.), ULTRA DISPER (ASADA IRON WORKS. Co., Ltd.), EBARA MILDER (manufactured by Ebara Corporation) may be used.

EXAMPLES

Hereinafter, this invention will be illustrated in detail by practical examples. However, it is to be understood that the examples are given for illustrative purposes only, and the invention is not limited thereto.

The respective physical properties illustrated later in the examples are measured by the following protocols.

Bulk Density

One gram (1 g) of powder was added into a transparent circular cylinder which has 70 mm of an inner diameter equipped with a distribution plate. Then, 1.3 liter of air at 0.1 Mpa of pressure was supplied from the lower side of the distribution plate in order to blow the powder loose. Afterwards, the powder was allowed to settle naturally. After the fifth air blowing, the height of the settled powder layer was measured. Six random data points were taken and averaged in order to determine the bulk density.

Raman Spectroscopic Analysis

Raman spectroscopic analysis was performed with LabRam 800 manufactured by HORIBA JOBIN YVON, S.A.S., using a 514 nm wavelength argon laser.

TG Combustion Temperature

Combustion behavior was determined using TG-DTA manufactured by MAX SCIENCE CO. LTD., at an air flow rate of 0.1 liter/minute and a heating rate of 10° C./minute. When combustion occurs, TG indicates a quantity reduction and DTA indicates an exothermic peak. Thus, the top position of the exothermic peak was defined as the combustion initiation temperature.

X Ray Diffraction

Using a powder X ray diffraction equipment (JDX3532, manufactured by JEOL Ltd.), the structures of carbon fibers after annealing processing were analyzed. Kα ray generated with a Cu tube at 40 kV, 30 mA was used, and measurement of the spacing was performed in accordance with the method defined by The Japan Society for the Promotion of Science (JSPS), described in “Latest Experimental Technique For Carbon Materials (Analysis Part),” Edited by Carbon Society of Japan, 2001. Silicon powder was used as an internal standard. The related parts of this literature are incorporated herein by reference.

Particle's Resistance and Decompressibility

One gram (1 g) of CNT powder was weighed out, and then press-loaded into a resinous die (inner dimensions: 40 L, 10 W, 80 Hmm). The displacement and load were read out. A constant current was applied to the powder using the four-terminal method, and the voltage was measured under this condition. After monitoring the voltage until the density came to 0.9 g/cm³, the applied pressure was released and the density after decompression was measured. Measurements taken when the powder was compressed to 0.5, 0.8 or 0.9 g/cm³ were adopted as the particle's resistance.

Coating Ability

This property was determined according to the following criteria.

∘: It is easy to coat by a bar coater.

×: It is difficult to coat by a bar coater.

Total Light Transmittance

Total light transmittance was determined in accordance with JIS K 7361, by using a haze/transmittance meter HM-150 (manufactured by MURAKAMI COLOR RESEARCH LABORATORY), for a coating film having a prescribed thickness formed on a glass plate (total light transmittance of 91.0%, 50×50×2 mm).

Surface Resistivity

A 50×50 mm coated harden film was prepared on a glass plate.

Using a 4-pin probe type resistivity meter (MCP-T600 or MCP-HT4500, both manufactured by Mitsubishi Chemical), the resistance (Ω) at nine points on the coated film surface was measured. The measured values were converted into volume resistivity (Ω·cm) by the resistivity meter. An average was then calculated.

Synthetic Example 1

Carbon fibrous structures were synthesized using toluene as a raw material in a CVD process.

The synthesis was carried out in the presence of a mixture of ferrocene and thiophene as a catalyst, and in the reducing atmosphere of hydrogen gas. The toluene and the catalyst were heated to 380° C. along with the hydrogen gas. The heated mixture was then supplied to a generation furnace and subjected to thermal decomposition at 1250° C. in order to obtain carbon fibrous structures (the first intermediate). The synthesized first intermediate was baked at 900° C. in nitrogen gas in order to remove hydrocarbons, such as tar, to produce a second intermediate. The R value of the second intermediate, as measured by Raman spectroscopic analysis, was found to be 0.98. A sample for electron microscopy was prepared by dispersing the first intermediate into toluene. FIG. 1 shows a TEM photo of the sample.

The second intermediate was further subjected to a high temperature heating treatment at 2600° C. in argon. The obtained aggregates of the carbon fibrous structures were pulverized using an air flow pulverizer in order to produce carbon fibrous structures according to the present invention. A sample for electron microscopy was prepared by dispersing ultrasonically the obtained carbon fibrous structures into toluene. FIGS. 2A and 2B show TEM photos of the sample.

X-ray diffraction analysis and Raman spectroscopic analysis were performed on the carbon fibrous structures before and after the high temperature heating treatment in order to examine the changes. The results are shown in FIGS. 3 and 4, respectively.

It was found that the carbon fibrous structures had a bulk density of 0.0032 g/cm³, a Raman _(D)/I_(G) ratio of 0.090, a TG combustion temperature of 786° C., a spacing of 3.383 angstrom, a particle's resistance of 0.0083 Ω·cm, and a density after decompression of 0.25 g/cm³.

Examples 1-9

The carbon fibrous structures obtained in Synthetic Example 1 was added to 100 parts by weight of polyurethane resin solution (non-volatile matter: 20%) at ratios shown in Table 1. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) with zirconium beads (0.05 mm, 0.5 mm, 1.0 mm, or 1.5 mm in diameter) at a peripheral speed of 10 m/sec, a bead filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibrous structures dispersed therein was prepared.

The liquid resinous composition obtained above was coated on a glass plate to obtain a hardened film of a prescribed thickness shown in Table 1. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 1.

Furthermore, the dispersion condition of the carbon fibrous structures in the hardened coating film was observed by an electron microscope. The result obtained is shown in FIG. 5.

Reference Examples 1-6

To prepare the coating compositions of Reference Examples 1-6, the same procedure in Examples 1-9 was repeated except that the dispersion method and its condition were changed as shown in Table 1. Then, the same tests for coating ability, total light transmittance, and surface resistivity as in Examples 1-9 were performed. The results obtained are shown in Table 1.

Controls 1-4

Multilayered carbon nanotubes (manufactured by Tsinghua Nafine, 10-20 nm in outer diameter, several μm to several tens μm in length) were added to 100 parts by weight of a polyurethane resin solution (non-volatile matter: 20%) at ratios shown in Table 1. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) with zirconium beads (0.05 mm, or 1.5 mm in diameter) at a peripheral speed of 10 m/sec, a bead filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibers dispersed therein was prepared.

The liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness shown in Table 1. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 1.

Examples 10-18

The carbon fibrous structures obtained in Synthetic Example 1 was added to 100 parts by weight of a polyester resin solution (non-volatile matter: 65%) at ratios shown in Table 2. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) with zirconium beads (0.05 mm, 0.5 mm, 1.0 mm, or 1.5 mm in diameter) at a peripheral speed of 10 m/sec, a beads filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibrous structures dispersed therein was prepared.

The liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness as shown in Table 2. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 2.

Reference Examples 7-12

To prepare the coating compositions of Reference Examples 7-12, the same procedure in Examples 10-18 was repeated, except that the dispersion method and its condition were changed as shown in Table 2. The same tests for coating ability, total light transmittance, and surface resistivity as in Examples 10-18 were performed. The results obtained are shown in Table 2.

Controls 5-8

Multilayered carbon nanotubes (manufactured by Tsinghua Nafine, 10-20 nm in outer diameter, several μm to several tens μm in length) was added to 100 parts by weight of a polyester resin solution (non-volatile matter: 65%) at ratios shown in Table 2. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) with zirconium beads (0.05 mm, or 1.5 mm in diameter) at a peripheral speed of 10 m/sec, a bead filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibers dispersed therein was prepared.

The liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness as shown in Table 2. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 2.

Examples 19-27

The carbon fibrous structures obtained in Synthetic Example 1 was added to 100 parts by weight of a phenolic resin (non-volatile matter: 50%) at ratios shown in Table 3. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) with zirconium beads (0.05 mm, 0.5 mm, 1.0 mm, or 1.5 mm in diameter) at peripheral speed of 10 m/sec, a bead filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibrous structures dispersed therein was prepared.

The liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness as shown in Table 3. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 3.

Reference Examples 13-18

To prepare the coating compositions of Reference Examples 13-18, the same procedure in Examples 19-27 was repeated, except that the dispersion method and its condition were changed as shown in Table 3. The same tests for coating ability, total light transmittance, and surface resistivity as in Examples 19-27 were performed. The results obtained are shown in Table 3.

Controls 9-12

Multilayered carbon nanotubes (manufactured by Tsinghua Nafine, 10-20 nm in outer diameter, several μm to several tens μm in length) was added to 100 parts by weight of phenolic resin (non-volatile matter: 50%) at ratios shown in Table 3. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) with zirconium beads (0.05 mm, or 1.5 mm in diameter) at a peripheral speed of 10 m/sec, a bead filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibers dispersed therein was prepared.

The liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness shown in Table 3. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 3.

Examples 28-36

The carbon fibrous structures obtained in Synthetic Example 1 was added to 100 parts by weight of acrylic resin (non-volatile matter: 35%) at ratios shown in Table 4. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) under the conditions of zirconium beads (0.05 mm, 0.5 mm, 1.0 mm, or 1.5 mm in diameter) at a peripheral speed of 10 m/sec, a beads filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibrous structures dispersed therein was prepared.

The liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness shown in Table 4. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 4.

Reference Examples 19-24

To prepare the coating compositions of Reference Examples 19-24, the same procedure in Examples 28-36 was repeated, except that the dispersion method and its condition were changed as shown in Table 4. Then, the same tests for coating ability, total light transmittance, and surface resistivity as in Examples 28-36 were performed.

The results obtained are shown in Table 4.

Controls 13-16

Multilayered carbon nanotubes (manufactured by Tsinghua Nafine, 10-20 nm in outer diameter, several μm to several tens μm in length) was added to 100 parts by weight of a acrylic resin (non-volatile matter: 35%) at ratios shown in Table 4. The resultant mixture was pulverized and dispersed by using a bead mill (DYNO-MILL, manufactured by SHINMARU ENTERPRISES CORPORATION) with zirconium beads (0.05 mm, or 1.5 mm in diameter) at a peripheral speed of 10 m/sec, a bead filling rate of 80% by volume, and a processing time of 2 hrs. As a result, a coating composition comprising the carbon fibers dispersed therein was prepared.

T the liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness shown in Table 4. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Table 4.

Examples 37-40

The same procedures in Examples 6, 15, 24, and 33 were repeated except that an additional dispersion treatment, using T.K. FILMICS® (manufactured by TOKUSHI KIKA KOGYO CO., LTD) at a tip speed of 50 m/sec for 2 minutes, was performed before the dispersion treatment using the bead mill. As a result, a coating composition comprising the carbon fibrous structures dispersed therein was prepared.

The liquid resinous composition thus obtained was coated on a glass plate to obtain a hardened film of a prescribed thickness shown in Tables 1-4. The hardened film was tested for coating ability, total light transmittance, and surface resistivity. The results obtained are shown in Tables 1-4. TABLE 1 Example 1 2 3 4 5 6 7 8 9 37 Polyurethane resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 5 1.5 2 2 5 5 25 25 25 5 structures Multilayered CNT Method Bead mill ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ dispersion Ball mill dispersion Homogenizer dispersion Pretreatment ∘ (high-shear type dispersion) Beads' diameter 1.0 0.05 0.05 1.5 0.5 1.0 0.5 1.0 1.5 1.0 (mm) Thickness(μm) 0.2 1.0 1.0 3.0 0.5 1.5 1.0 1.0 4.5 1.5 Coating ability ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Total light 88.5 90.2 87.5 77.3 83.5 70.5 52.5 52.8 33.8 78.4 transmittance Surface resistivity 3.8 × 10⁸ 2.7 × 10¹¹ 3.6 × 10¹⁰ 2.4 × 10⁹ 5.8 × 10⁷ 4.7 × 10⁶ 1.2 × 10² 3.7 × 10² 2.6 × 10¹ 1.1 × 10⁶ Reference Control 1 2 3 4 5 6 1 2 3 4 Polyurethane resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 1.5 0.5 35 5 5 5 structures Multilayered CNT 1.5 1.5 5 5 Method Bead mill ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ dispersion Ball mill dispersion ∘ Homogenizer ∘ dispersion Pretreatment (high-shear type dispersion) Beads' diameter 0.03 0.05 0.05 2.0 0.5 0.5 0.05 1.5 0.05 1.5 (mm) Thickness(μm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Coating ability ∘ ∘ X ∘ ∘ ∘ ∘ ∘ ∘ ∘ Total light 89.4 90.5 ND ND ND ND 22.3 17.5 ND ND transmittance Surface resistivity >10¹² >10¹² 2.9 × 10¹ 2.1 × 10⁶ 9.8 × 10⁵ 3.7 × 10⁶ >10¹² >10¹² 3.5 × 10⁹ 1.3 × 10⁹ ND: not determined multilayered CNT: multilayered carbon nanotube (manufactured by Tsinghua Nafine, 10-20 nm in outer diameter, several μm to several tens μm in length)

TABLE 2 Example 10 11 12 13 14 15 16 17 18 38 Polyester resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 5 1.5 2 2 5 5 25 25 25 5 structures Multilayered CNT Method Bead mill ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ dispersion Ball mill dispersion Homogenizer dispersion Pretreatment ◯ (high-shear type dispersion) Beads' diameter 1.0 0.05 0.05 1.5 0.5 1.0 0.5 1.0 1.5 1.0 (mm) Thickness(μm) 0.2 1.0 1.0 3.0 0.5 1.5 1.0 1.0 4.5 1.5 Coating ability ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ Total light 83.8 89.2 84.5 73.8 83.8 69.8 52.4 51.7 33.2 74.8 transmittance Surface resistivity 6.8 × 10⁸ 4.2 × 10¹¹ 5.1 × 10¹⁰ 3.4 × 10⁹ 2.8 × 10⁷ 4.3 × 10⁶ 3.2 × 10² 5.3 × 10² 4.5 × 10¹ 2.7 × 10⁶ Reference Control 7 8 9 10 11 12 5 6 7 8 Polyester resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 1.5 0.5 35 5 5 5 structures Multilayered CNT 1.5 1.5 5 5 Method Bead mill ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ dispersion Ball mill dispersion ◯ Homogenizer ◯ dispersion Pretreatment (high-shear type dispersion) Beads' diameter 0.03 0.05 0.05 2.0 0.5 0.5 0.05 1.5 0.05 1.5 (mm) Thickness(μm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Coating ability ◯ ◯ X ◯ ◯ X ◯ ◯ ◯ ◯ Total light 87.5 90.2 ND ND ND ND 24.5 20.1 ND ND transmittance Surface resistivity >10¹² >10¹² 5.3 × 10¹ 3.2 × 10⁶ 8.6 × 10⁵ 4.3 × 10⁶ >10¹² >10¹² 4.4 × 10⁹ 2.8 × 10⁹ ND: not determined multilayered CNT: multilayered carbon nanotube (manufactured by Tsinghua Nafine, 10-20 nm in Outer diameter, several μm to several tens μm in length)

TABLE 3 Example 19 20 21 22 23 24 25 26 27 39 Phenolic resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 5 1.5 2 2 5 5 25 25 25 5 structures Multilayered CNT Method Bead mill ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ dispersion Ball mill dispersion Homogenizer dispersion Pretreatment ◯ (high-shear type dispersion) Beads' diameter 1.0 0.05 0.05 1.5 0.5 1.0 0.5 1.0 1.5 1.0 (mm) Thickness(μm) 0.2 1.0 1.0 3.0 0.5 1.5 1.0 1.0 4.5 1.5 Coating ability ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ Total light 82.1 89.5 81.2 74.8 82.4 65.2 51.9 50.3 33.6 72.5 transmittance Surface resistivity 4.9 × 10⁸ 4.2 × 10¹¹ 5.7 × 10¹⁰ 4.1 × 10⁹ 7.3 × 10⁷ 5.2 × 10⁶ 3.2 × 10² 5.2 × 10² 8.6 × 10¹ 1.6 × 10⁶ Reference Control 13 14 15 16 17 18 9 10 11 12 Phenolic resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 1.5 0.5 35 5 5 5 structures Multilayered CNT 1.5 1.5 5 5 Method Bead mill ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ dispersion Ball mill dispersion ◯ Homogenizer ◯ dispersion Pretreatment (high-shear type dispersion) Beads' diameter 0.03 0.05 0.05 2.0 0.5 0.5 0.05 1.5 0.05 1.5 (mm) Thickness(μm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Coating ability ◯ ◯ X ◯ ◯ X ◯ ◯ ◯ ◯ Total light 88.7 89.6 ND ND ND ND 18.6 13.8 ND ND transmittance Surface resistivity >10¹² >10¹² 4.2 × 10¹ 3.4 × 10⁶ 1.6 × 10⁶ 5.3 × 10⁶ >10¹² >10¹² 3.9 × 10⁹ 2.3 × 10⁹ ND: not determined multilayered CNT: multilayered carbon nanotube (manufactured by Tsinghua Nafine, 10-20 nm in outer diameter, several μm to several tens μm in length)

TABLE 4 Example 28 29 30 31 32 33 34 35 36 40 Acrylic resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 5 1.5 2 2 5 5 25 25 25 5 structures Multilayered CNT Method Bead mill ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ dispersion Ball mill dispersion Homogenizer dispersion Pretreatment ◯ (high-shear type dispersion) Beads' diameter 1.0 0.05 0.05 1.5 0.5 1.0 0.5 1.0 1.5 1.0 (mm) Thickness(μm) 0.2 1.0 1.0 3.0 0.5 1.5 1.0 1.0 4.5 1.5 Coating ability ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ Total light 87.2 88.5 84.8 76.2 85.1 66.4 54.3 53.7 34.6 76.5 transmittance Surface resistivity 5.2 × 10⁸ 1.4 × 10¹¹ 1.9 × 10¹¹ 2.1 × 10⁹ 3.5 × 10⁷ 2.5 × 10⁶ 1.8 × 10² 4.5 × 10² 4.8 × 10¹ 1.2 × 10⁶ Reference Control 19 20 21 22 23 24 13 14 15 16 Acrylic resin 100 100 100 100 100 100 100 100 100 100 Carbon fibrous 1.5 0.5 35 5 5 5 structures Multilayered CNT 1.5 1.5 5 5 Method Bead mill ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ dispersion Ball mill dispersion ◯ Homogenizer ◯ dispersion Pretreatment (high-shear type dispersion) Beads' diameter 0.03 0.05 0.05 2.0 0.5 0.5 0.05 1.5 0.05 1.5 (mm) Thickness(μm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Coating ability ◯ ◯ X ◯ ◯ X ◯ ◯ ◯ ◯ Total light 90.2 89.5 ND ND ND ND 32.1 28.7 ND ND transmittance Surface resistivity >10¹² >10¹² 5.1 × 10¹ 3.6 × 10⁶ 5.3 × 10⁵ 2.4 × 10⁶ >10¹² >10¹² 5.5 × 10⁹ 3.3 × 10⁹ ND: not determined multilayered CNT: multilayered carbon nanotube (manufactured by Tsinghua Nafine, 10-20 nm in outer diameter, several μm to several tens μm in length)

The present invention may be embodied in other specific forms without departing from the scope or essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 

1. A transparent conductive film, comprising a matrix resin and carbon fibrous structures added to the matrix, wherein the carbon fibrous structures comprise carbon fibers, each having an outside diameter of 15-100 nm, and wherein the carbon fibrous structures each comprise a granular part at which two or more carbon fibers are bound to each other, and wherein the granular part is produced in a growth process for the carbon fibers.
 2. The transparent conductive film according to claim 1, wherein the carbon fibrous structures are added at an amount in the range of 1 to 25 parts by weight based on 100 parts by weight of the matrix.
 3. The transparent conductive film according to claim 1, wherein the transparent conductive film has a surface resistivity of not more than 1.0×10¹²Ω/□, and a total light transmittance of not less than 30%, when the transparent conductive film is formed at a thickness of 0.1-5 μm on a glass substrate.
 4. A coating composition for a transparent conductive film, comprising a liquid resinous composition and carbon fibrous structures dispersed into the resinous composition, wherein the carbon fibrous structures comprise carbon fibers each having an outside diameter of 15-100 nm, and wherein the carbon fibrous structures each comprise a granular part at which two or more carbon fibers are bound to each other, and wherein the granular part is produced in a growth process for the carbon fibers.
 5. The coating composition according to claim 4, wherein the resinous composition comprises a resin as a non-volatile vehicle.
 6. The coating composition according to claim 4, wherein the carbon fibrous structures are added at an amount in the range of 1 to 25 parts by weight based on 100 parts by weight of the liquid resinous composition.
 7. The coating composition according to claim 4, wherein the coating composition is prepared by using a media mill equipped with beads having an average diameter of 0.05-1.5 mm to disperse the carbon fibrous structures into the liquid resinous composition.
 8. The coating composition according to claim 7, wherein the coating composition is prepared by using a high-shear type distributor prior to dispersing the carbon fibrous structures using the media mill. 